Please note that the tag was initially incorrect.
The E stereo isomer is the higher energy form; the Z form is calculated to be 3.563 kJ/mol lower in energy. The rotational spectrum was recorded between 81 and 374 GHz by
(1) L. Bizzocchi, D. Prudenzano, V. M. Rivilla, A. Pietropolli-Charmet, B. M. Giuliano, P. Caselli, J. Martín-Pintado, I. Jiménez-Serra, S. Martín, M. A. Requena-Torres, F. Rico-Villas, S. Zeng, and J.-C. Guillemin, 2020, Astron. Astrophys. 640, Art. No. A98.
The analysis takes into account lower frequency data from
(2) M. Sugie, H. Takeo, and C. Matsumura, 1985, J. Mol. Spectrosc. 111, 83.
The parameter set differs slightly from the one in (1). The calculations should be sufficiently accurate for all observational purposes. Frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with caution.
¹⁴N hyperfine splitting may be resolvable in could sources at low quantum numbers. Therefore, a separate hyperfine calculation is provided for J ≤ 20 below 180 GHz. NOTE: The partition function does take into account the spin multiplicity of the ¹⁴N nucleus !
The two stereo isomeric forms were treated as separate species in (1) which is certainly appropriate at low temperatures. The partition function below refers to the ground vibrational state only. Vibrational correction factors for a posteriori corrections were derived in (1) in the harmonic approximation.
It may be useful to treat the two forms as being in local thermodynamic equilibrium at higher temperatures. Corresponding frequency calculations along with vibrational correction factors with contributions for E for a posteriori corrections of the rotational partition function of Z were derived in the harmonic approximation.
The dipole moment components are from a quantum-chemical calculation in (1).